It is known in the art to mount a speed sensor in the transmission or in the bearing or wheel end assembly of a vehicle such as a truck or an automobile. The sensor measures the angular velocity of a rotating element within the transmission or bearing assembly. Typically, sensors of this type utilize variable reluctance, a form of electromagnetic sensing, generally known and understood by those skilled in the art. Such sensors are used in transmissions to assist in electronic shifting and in vehicle anti-lock braking (ABS) systems to detect wheel speed.
Generally, sensors of this type include a coil mounted on a bobbin which is inserted into a housing. A wheel speed sensor for use in an ABS application typically is connected or bolted into a hub assembly which is located by a brake, usually a front brake, of a vehicle. A tone wheel rotates with or around an axle, typically a front axle, and induces a voltage signal within the wheel speed sensor. The wheel sensor communicates with an on-board controller or computer of the vehicle when any of the vehicle's four wheels lock-up in a panic breaking situation. The sensor sends a voltage signal to the computer and the computer determines if a correct voltage signal has been sent. Based on the signal received, if the computer determines that any of the wheels of the vehicle has locked-up, the vehicle computer automatically takes over and controls the vehicle braking system, pumping each brake for a safer controlled, skid-free stop.
The environment in which sensors of this type are subjected can be extremely harsh and, as a result, the sensors must be capable of withstanding a significant amount of repeated temperature variations over a wide range of temperatures (thermal cycling), intense vibration and exposure to corrosive elements. One such wheel speed sensor is described in commonly assigned U.S. Pat. No. 5,629,618, which is hereby incorporated by reference. In the '618 patent, a wheel speed sensor is described such that in order to seal the sensor from the environment, a bobbin is connected to a housing and the combination is overmolded with an injected molded plastic cover, i.e., an overmold, to provide a molded seal between the bobbin and the housing.